TiNi as a nano-actuator: experimental verification
of excitation by electron-beam heating

A. D. Johnson,
M. Fanucchi, V. Gupta, V. Martynov, V.Galhotra

TiNi Alloy Company
Inc., 1621 Neptune Drive, San Leandro CA 94577

K. Clements

Innovation On Demand
Inc.

Abstract

High work output
per unit volume makes TiNi actuators attractive as a means of powering
nano-robotic movement. Such devices are being developed for manipulation
of structures at near the molecular scale. In these very small devices
(one micron scale), one means of delivery of energy is by electron beams.

Movement of mechanical
structures a few microns in extent has been demonstrated in a scanning
electron microscope. Results of these and subsequent experiments
will be described, with a description of potential structures for fabricating
moving a microscopic x-y stage.

Overview

Miniaturization of
mechanical devices is evolving toward nanometer scale, requiring handling
and assembly of objects as small as a few nanometers. Manipulation of samples
and specimens smaller than a few microns in size demands a technology that
at present does not exist. Assemblers are needed that can grip collections
of molecules, releasing them from their present location, lifting, rotating,
and forcefully placing them in a new environment.

Existing micropositioners
do not provide the requisite flexibility of motion for assembly tasks that
are contemplated.

Forceful shape memory
alloy actuators can be scaled to micron size. These devices are thermally
powered and so require a source of heat energy: this heat may be supplied
by conduction, joule heating, infrared light, or other means.

We have started to
explore use of a scanning electron microscope beam to provide heat energy
to energize thermal actuators. Prototype actuators are fabricated
by sputter deposition of titanium-nickel thin film, photolithographic patterning,
and chemical milling. A scanning electron beam is positioned to produce
local heating, and to observe the resulting motion.

Background

In 1982 K. Eric Drexler
introduced the idea of assemblers of molecular size in his book “Engines
of Creation”.[1] Nanotechnology is the subject of at least one international
conference, and one commercial venture has been organized and funded to
invest research and planning in this technology. [2] Although commercial
realization of nanotechnology may be years away, there is strong indication
that research following the human genome project, and particularly the
study of protein structure and function, will require tools to manipulate
components of the cell. Development of these tools is a demanding,
exciting, and challenging research subject.

Atomic force microscopy
can be used to move individual atoms but not to grip larger objects with
enough force to hold against local forces. In recent investigations
of the properties of carbon nanotubes, piezoelectric stepper motors have
been used to manipulate structures orders of magnitude smaller than the
drivers.[3]

Manipulation of objects
this small would be improved if the end-effectors were not much larger
than the objects they control. In analogy to the shoulder-wrist-finger
arrangement of the human hand, gross positioning should be managed by actuators
of macroscopic size, and fine control by end-effectors of much smaller
size.

The force of actuation
should be produced as close as possible to the point of application.
This implies that manipulation of sub-micron size objects requires micron-size
actuators. Conventional actuators (electromagnetic, piezoelectric) do not
scale well to micron size. A promising form of actuation is heat-actuated
devices, particularly shape memory actuators. [4] Photolithography
provides means of fabricating devices of sub-micron size. Miniature
shape memory alloy (SMA) actuators rely on joule heating to cause the phase
change. In the sub-micron range it is difficult to make electrical
connection, especially on devices that move. To solve this problem,
we have chosen to actuate sub-micron scale shape memory alloy devices
by electron-beam excitation.

Current devices are
fabricated as small as a few hundred microns using conventional microlithography.
Shrinking this technology to sub-micron dimensions raises at least two
questions:
(i) Will the shape memory property be preserved when the dimensions
are as small or smaller than the crystal domains? And
(ii) How can such small objects (sub-micron) be selectively
heated to produce actuation? This research effort was undertaken
to provide preliminary proof-of-concept answers.

Verification of Phase Change

At this conference
one year ago, the question was raised: how small can shape memory thin
film be shown to demonstrate the shape memory property.[5] A partial
answer to this question was obtained: a film approximately 100 nm (about
200 atomic layers) thick was shown to undergo a phase transformation as
indicated by a change in resistivity. This result is shown in Fig.
1. The change in slope and hysteresis loop are typical for TiNi shape memory
alloy.

Figure 1: Resistivity versus temperature
for TiNi film sputter-deposited on silicon oxide. Film thickness
is approximately 100 nm.

Verification of shape recovery

A scanning electron
microscope e-beam may be used to provide the energy, and beam steering
can bring a spot of energy a fraction of a micron in diameter to bear on
a sample. How much power must be delivered to the sample, and can
the beam provide this much energy in a short enough time to effect a shape
change?

The estimated energy
required to actuate a TiNi specimen 4 x 10 x 100 microns by heating it
from the room temperature to the transition point is about 1.3 x 10 –5
joules (DT
= ~80 C, DH
= ~25 j/gm, Cp = ~0.3 J/gmoC,
density = 6.4 gm/ cm3). The power available from the electron
beam is 2x10-3 watt (for accelerating voltage ~20 KV the beam
current ~10-7 A) and the estimated heating time is ~6x10-3
sec.

Demonstration of
shape recovery requires that the specimen be pre-strained (stretched, compressed,
or bent) while it is in its low-temperature state and then heated above
the phase transformation temperature. The sample used is a fragment of
TiNi film 4 micrometers thick deposited on silicon oxide, patterned with
fenestrations about 40 microns in diameter, and removed from the substrate.
This film was further etched to diminish the width and thickness of the
web elements. The resulting web was torn apart, producing small protrusions
about 1-2 micron wide and 20 microns long. Some of these were bent
during tearing, others were deformed manually using a micromanipulator.
One such structure is shown in Figure 2.

Figure2.
Scanning electron beam image of TiNi film with fenestrations. The
wider web structures are about 20 microns wide.

The specimen was placed
on a heated pedestal in an ISI60 scanning electron microscope. The
pedestal was equipped with a heater and thermocouple so that the base temperature
could be controlled and measured. This fixture is shown in Figure3.
Fluke instruments were used to record temperature and current through the
heater, and an IRF 640 field-effect transistor with a variable gate voltage
was used to control the current through the heater to vary the temperature
of the substrate.

Figure 3:
Heated sample holder for SEM.

The chamber was evacuated
and the beam was started. A picture was obtained at 1.5 kx magnification.
The sample holder was heated with resistance heater to a temperature above
ambient and below the transition temperature of the TiNi. This was
to enable the electron beam to bring the temperature through a relatively
small temperature change to effect the phase transformation. The
beam was centered approximately on the bent portion of the microbeam.

The SEM beam aperture
was opened to impart the maximum current to the specimen, using spot mode,
and current in the beam was increased. Typical current used was 70 to 100
nano-amperes measured with a Kiethley picoammeter connected between the
sample and ground potential. This current was applied to the sample
for between 2 and 10 seconds. After exposure, the beam current was
reduced and further pictures taken.

The results are shown in Figure 4 (a through
c). The first picture shows the sample previous to heating.
Images b,c show the progressive actuation as successive parts of the device
were heated by electron beam. Approximately 30 degrees of recovery was
achieved. The lever is about 2 microns in diameter and 20 microns
long.

a)
b)
c)

Figure 4:Sequence
of images from the SEM showing the results of heating the TiNi specimen
by SEM electron beam. Partial actuation was achieved as evidenced
by motion of the tip of the lever.

Conclusions

The micro-cantilever
moving about 30 degrees from the original position was observed in the
experiment . This was not due to thermal expansion as it did not
reverse when the temperature was reduced. Thus actuation of a micro-scale
device by scanning electron beam has been demonstrated, showing that the
e-beam can provide enough energy to cause the phase transformation under
controlled conditions.

It
should be possible to construct a platform having x-y motion by placing
pairs of opposed bending cantilevers in each direction,so that partial
actuation of one cantilever pushes the platform while pre-straining the
opposing cantilever. Larger-scale translational motion can be achieved
with multiple actuators operating in sequence against a ratchet.